Predicting the Anticoagulant Activity of Apixaban from Prothrombin Time of Warfarin when Replacing Warfarin with Apixaban

Background: Warfarin has a narrow therapeutic window, interacts with some foods, and requires routine monitoring. Direct oral anticoagulants (DOACs) are able to resolve some of the drawbacks of warfarin. Recently, it has been reported that the incidence of major bleeding in patients with atrial fibrillation who are receiving apixaban is similar to that in patients receiving warfarin. We therefore sought indices formeasuring/monitoringthe anticoagulant activity of apixaban. Methods: Twenty-three patients (70.0±8.6years;17 males, 6 females) with nonvalvular atrial fibrillation, for whom warfarin was replaced with 10mg/day apixaban, were enrolled in the studyfrom February 1, 2013 to July 31, 2014. We examined the relationship between prothrombin time of warfarin (PTw) and PT of apixaban (PTa) by using paired t-test. We also used multiple regression analysis to predict PTa from PTw and other factors (age, body surface area, serum creatinine, and creatinine clearance). Results: PTw was significantly shorter than PTa (PTw, 19.0±3.2s versus PTa, 15.5±1.5s; P< 0.0001), and PTa was significantly prolonged compared with the normal PT range of 10s to 12s (P = 0.01). This finding suggested that PT is sensitive in the prediction of apixaban-associated Xa activity, and PTa could be predicted using PTw as follows: predicted PTa = 10.515 + 0.263×PTw (P = 0.056). Conclusions: When warfarin was replaced with 10mg/day apixaban, PTa could be predicted using PTw without any routine monitoring to evaluate anticoagulant activity.